1. Field of the Invention
The present invention relates generally to sealed bearing earth boring drill bits, such as rotary cone rock bits that utilize a fluid circulation medium. More particularly, the invention relates to such drill bits that have a dual seal arrangement for protecting internal bearing elements.
2. Background
During earthen drilling operations with the use of sealed bearing rotary cone drill bits, it is necessary to protect the bearing elements of the bit from contamination in order to sustain bit operability. In particular, it is desirable to isolate and protect the bearing elements of the bit, such as bearings, lubricant and bearing surfaces that are located in a cavity or cavities between each corresponding bit leg and cutter cone, from earthen cuttings, mud and other debris in the drilling environment. Introduction into the bearing system of such contaminants leads to deterioration of the bearing lubricant, bearings and bearing surfaces, causing premature bit failure.
It is well known in the art to provide an annular seal around the bearing elements to prevent contamination thereof by particles entering from the exterior of the bit through the annular opening and gap formed between each leg and corresponding cutter cone and extending to the cavity or cavities carrying bearing system elements. Single seal arrangements, however, are known to be susceptible to leakage and premature wear due, for example, to the intrusion and abrasive effect of particles entering through the annular opening. When the single seal fails to provide an absolute seal, the bearing elements become contaminated.
Dual seal arrangements have been proposed having an annular outer seal disposed around a primary inner seal for providing a second layer of protection from particles entering the annular opening. Generally, the outer seals of many prior art dual seal arrangements have proven to provide insufficient protection around the primary seal. When the outer seal fails and allows particles or other contaminants to the primary seal, contamination of the bearing elements can occur as described above. Furthermore, various prior art dual seal arrangements require additional components or internal bit mechanisms that are subject to malfunctioning, increasing the risk of contamination of the bearing system, or that increase the complexity of the bit and its manufacturing process.
U.S. Pat. No. 3,746,405, for example, discloses a sealed bearing rotary cone rock bit with a dual O-ring seal arrangement. The outer O-ring seal is disposed in a groove formed in the cone for engaging the leg backface, and is thus being primarily axially energized. During operation, the use of an outer O-ring seal that is axially energized is highly susceptible to failure, or lift-off, as the cone moves relative to the leg during the normally occurring misalignment, or end-play of the cone, allowing the ingress of particles from the annular opening to the inner seal.
Attempts have been made to prevent particles from entering the annular opening by directing pressurized drilling fluids, such as air, from within the drill bit out through the annular opening. U.S. Pat. No. 4,102,419 discloses a rock bit with an inner metal sealing ring and a set of outer hardened mating surfaces between the leg and cone. Fluid is directed from inside the bit, by the inner metal sealing ring and the outer mating surfaces, and through the annular opening to flush cuttings or other debris away from the inner seal. If the exiting fluid flow decreases or ceases, however, the outer mating surfaces will not effectively prevent the ingress of particles to the inner metal sealing ring due to the relative movement between the cone and leg during cone misalignment, thus exposing the inner metal sealing ring to contamination.
It has also been proposed to include a groove formed in the cone between the inner primary seal and the annular opening for directing fluid from inside the bit through the annular opening to prevent the entry of particles from the borehole. In U.S. Pat. No. 4,453,836, air is supplied into a groove formed between the cone and leg. An outer labyrinth-type seal is disposed in a groove formed in the leg journal segment between the inner seal and the air groove. The outer seal thus surrounds the inner seal, which is located in another groove in the leg journal segment. The disclosed labyrinth-type seal will, however, be susceptible to fluid/particle penetration if the exiting air flow decreases or ceases, leading to contamination of the inner seal.
U.S. Pat. No. 5,513,711 discloses a perforated outer Bellville spring seal disposed in a groove between an inner O-ring seal and the annular opening. Air ejected between the seals passes through the perforations in the outer seal and exits the bit through the annular opening for preventing the ingress of debris from the borehole. Again, should the air pressure decrease or cease, particles will likely pass through the perforations of the outer seal and ingress to the inner seal.
It is thus believed that prior art dual seal arrangements have significant disadvantages, and there remains a need for an effective dual seal arrangement for a sealed bearing rotary cone drill bit. The arrangement must include an annular outer seal capable of substantially protecting the annular inner seal around its circumference from particles that may enter the bit through the annular opening between the cutter cone and leg. The arrangement must also include an inner seal capable of effectively sealing the bearing system and preventing bearing system lubricant contamination. Both seals must be capable of operating under typical bit operating conditions, which include cone misalignment, or end-play, relative to the leg. Especially well received would be a dual seal arrangement capable of having a long expected life-span, occupying minimal space in the limited bit area surrounding the bearing system, and which would not include significant additional internal bit mechanisms or manufacturing requirements.